A procedure is described, for example, in European Patent No. EP 793 803, in which a crankshaft angle range designated as a segment is assigned to each cylinder of the internal combustion engine. The segment time in which the crankshaft sweeps over this angle range depends, among other things, on the energy converted in the combustion cycle. Sporadic misfires lead to an increase in segment times registered synchronously with ignition. A misfire may prolong one or more segment times. A measure for the irregular operation of the internal combustion engine is calculated by a suitable determination of filter coefficients of a programmable digital filter, the filtering being able to extend over an angular range of up to four camshaft revolutions of the internal combustion engine.
In PCT Publication No. WO 90/02874 a procedure of this type is described in which a misfire is detected with the aid of a lambda signal made available by a lambda sensor. The lambda sensor has a great sensitivity with respect to lambda changes in range of a stoichiometric combustion. A misfire expresses itself by a change in the lambda signal, because of a short-range increase in the oxygen concentration in the exhaust gas connected with it. The misfire may be allocated to a specific cylinder by consideration of the gas running time from the cylinder to the lambda sensor.
Since a misfire leads to the appearance of uncombusted fuel in the exhaust duct of the internal combustion engine, an exhaust gas treatment device that is present there may be damaged by an inadmissibly high temperature increase which may appear within the scope of an exothermic reaction that may possibly be taking place. If misfires are detected, suitable measures may be taken up for protecting the exhaust gas treatment device from inadmissibly high temperatures. For example, the fuel supply to the affected cylinder may be eliminated. The appearance of sporadic misfires in a cylinder or a change in regular combustion processes and misfires leads to a detectable increase in irregular operation, so that the misfires are able to be detected using the known methods. An increased irregular operation does not, however, appear if the combustions misfire in all cylinders within a time that lies within the range of seconds. If, for example, the fuel pressure drops down to ten percent, an air ratio lambda of more than 3 may appear, at which combustion of fuel in an Otto internal combustion engine becomes impossible. Because of the heating energy contained in the uncombusted fuel, the temperature in a catalytic converter connected downstream from the internal combustion engine may increase in a few seconds from the operating temperature of, for example, 500° C. to 1300° C., for instance. At this temperature, a thermal destruction of the catalyst structure can no longer be excluded.
The present invention is based on the object of providing a method for detecting misfires of an internal combustion engine and a device for carrying out the method, which are especially conditioned on lack of fuel and/or on errors in the fuel supply of the internal combustion engine.